Editing Synapsida (section)

==Linnaean and cladistic classifications==

At the turn of the 20th century, synapsids were thought to be one of the four main subclasses of [[reptile]]s. However, this notion was disproved upon closer inspection of skeletal remains, as synapsids are differentiated from reptiles by their distinctive temporal openings. These openings in the [[skull]] bones allowed the attachment of larger jaw muscles, hence a more efficient bite.

Synapsids were subsequently considered to be a later reptilian lineage that became mammals by gradually [[evolution|evolving]] increasingly mammalian features, hence the name "mammal-like reptiles" (also known as [[pelycosaur]]s). These became the traditional terms for all [[Paleozoic]] (early) synapsids. More recent studies have debunked this notion as well, and reptiles are now classified within [[Sauropsida]] (sauropsids), the sister group to synapsids, thus making synapsids their own [[taxonomy|taxonomic]] group.<ref name=Carroll397/><ref name=Benton122/>

As a result, the [[paraphyletic]] terms "mammal-like reptile" and "pelycosaur" are seen as outdated and disfavored in technical literature, and the term '''stem mammal''' (or sometimes '''protomammal''' or '''paramammal''') is used instead. [[Phylogenetically]], it is now understood that synapsids comprise an independent branch of the [[tree of life (biology)|tree of life]].<ref>{{cite journal |last = Angielczyk |first = Kenneth D. |year = 2009 |title = Dimetrodon is not a dinosaur: Using tree thinking to understand the ancient relatives of mammals and their evolution |journal = Evolution: Education and Outreach |volume = 2 |issue = 2 |pages = 257–271 |s2cid = 24110810 |doi = 10.1007/s12052-009-0117-4 |doi-access = free}}</ref> The [[monophyly]] of Synapsida is not in doubt, and the expressions such as "Synapsida contains the mammals" and "synapsids gave rise to the mammals" both express the same phylogenetic hypothesis. This terminology reflects the modern [[cladistic]] approach to animal relationships, according to which the only valid groups are those that include all of the descendants of a common ancestor: these are known as [[monophyletic]] groups, or [[clade]]s.

Additionally, [[Reptilia]] (reptiles) has been revised into a monophyletic group and is considered entirely distinct from Synapsida, falling within [[Sauropsida]], the sister group of Synapsida within Amniota.<ref name="modestoanderson2004">{{cite journal|last1=Modesto|first1=S.P.|last2=Anderson|first2=J.S.|year=2004|title=The phylogenetic definition of Reptilia|journal=Systematic Biology|volume=53|issue=5|pages=815–821|doi=10.1080/10635150490503026|pmid=15545258|doi-access=free}}</ref>

===Primitive and advanced synapsids===
The synapsids are traditionally divided for convenience, into [[therapsid]]s, an advanced group of synapsids and the branch within which mammals evolved, and stem mammals, (previously known as [[pelycosaur]]s), comprising the other six more primitive families of synapsids.<ref>[[Michael J. Benton|Benton, Michael J.]] (2005).'' [[Vertebrate Paleontology (Benton)|Vertebrate Paleontology]]'', 3rd ed. Oxford: [[Blackwell's|Blackwell Science Ltd]]. {{ISBN|0-632-05637-1}}. p. 120.</ref> Stem mammals were all rather lizard-like, with sprawling gait and possibly horny [[scute]]s, while therapsids tended to have a more erect pose and possibly hair, at least in some forms. In traditional taxonomy, the Synapsida encompasses two distinct [[evolutionary grade|grades]]: the low-slung stem mammals have given rise to the more erect therapsids, who in their turn have given rise to the mammals. In traditional vertebrate classification, the stem mammals and therapsids were both considered [[order (biology)|orders]] of the subclass Synapsida.<ref name=Romer/><ref name=Carroll397/>

===Practical versus phylogenetic usage of "synapsid" and "therapsid"===
In [[phylogenetic nomenclature]], the terms are used somewhat differently, as the daughter clades are included. Most papers published during the 21st century have treated "Pelycosaur" as an informal grouping of primitive members. Therapsida has remained in use as a clade containing both the traditional therapsid families and mammals.

Although Synapsida and Therapsida include modern mammals, in practical usage, those two terms are used almost exclusively when referring to the more [[basal (phylogenetics)|basal]] members that lie outside of [[Mammaliaformes]].